1. Field of the Invention
The present invention pertains to electron guns producing a cylindrical electron beam. It pertains more particularly to guns working under high voltage. These guns are used notably in longitudinal interaction electron tubes. These tubes are called "O" type tubes. In this type of tube, the electron beam is focused by a magnetic field colinear with the path of the electrons. Klystrons, travelling wave tubes, are "O" type tubes. It is possible to use these guns in other devices under vacuum, such as particle accelerators.
2. Description of the Prior Art
An electron gun producing a long and narrow electron beam is generally built around an axis of revolution. It has a cathode, generally made of a thermo-emissive material, heated and carried to a generally negative potential. It is surrounded by a focusing electrode known as a wehnelt cylinder, carried to the same potential as the cathode. The cathode emits an electron beam towards an anode. The wehnelt cylinder causes the beam of electrons coming from the cathode to converge. These two electrodes are surrounded by the anode. The anode and the wehnelt cylinder are opened at their center to let through the electron beam coming from the cathode. Grids may be inserted between the cathode and the wehnelt cylinder. Ceramic elements, with a cylindrical shape for example, act as a support for the electrodes and insulate them electrically from one another. The electron beam, emitted by the cathode and focused by the wehnelt cylinder and the anode, then penetrates a tunnel-shaped part which is the body of the electron tube. This body is generally grounded. The anode may be either carried to an intermediate potential between that of the cathode and that of the body of the tube, or carried to the same potential as the body of the tube. Within the body of the tube, the beam is focused by means of a solenoid, a permanent magnet or a sequence of alternating contiguous magnets. The body of the tube ends in an electron collector.
In order to obtain a homogeneous electron beam, having a desired diameter and little undulation, it is necessary to adjust the flux of the magnetic field that applies all along the electron beam, in the gun and in the body of the tube. The undulation of the electron beam is due to the effects of mutual repulsion of the electrons. At the cathode, the induction must be weak so as not to disturb the emission of electrons. It is increased as and when the distance from the cathode is increased in order to make the electron beam converge in the gun. Finally, the induction is given a constant value outside the gun, that is, in the body of the tube.
To prevent a excessively intense magnetic field from prevailing in the immediate vicinity of the cathode, a pole piece is generally placed between the gun and the body of the tube. This piece forms a screen with respect to the strong field present in the body of the tube. To obtain a beam having a desired with little undulation at the outlet of the gun, an appropriate compromise has to be made between the induction on the cathode, the induction in the body of the tube, the radius of the beam and its undulation. These configurations of the magnetic field are of vital importance and certain solutions have been provided for the problems related thereto:
The pole piece can be given a particular geometry. It is generally made of mild steel. From one piece to another, it may be variably open, variably thick and variably conical. However, the main action of the pole piece on the electron beam is only at the outlet of the gun. It has little effect at the cathode.
The gun may also be shielded by placing a cylindrical shield, made of mild steel, outside the gun. This shield is placed around ceramic elements, at the cathode but outside the gun. It is also possible to add a small solenoid inside the shield. This enables the adjustments during tests to be made more precise. In the case of a gun used in high-power and/or low frequency tubes, the electrodes are taken to high frequencies and there is a big space between them, in order to prevent jump sparks. Consequently, the gun has a large diameter and the shield will therefore also have a large diameter. It will be relatively distant from the cathode and its influence on the magnetic field at the cathode will therefore be weak, even if a small complementary solenoid has been added.
The present invention seeks to overcome these drawbacks and proposes an electron gun provided with a device producing a magnetic field in the vicinity of the cathode.